Plasma arc torches are used in industry for cutting metal because of their ability to produce concentrated heat. Typically, a plasma arc torch comprises a nozzle assembly comprising an electrode on which a power supply places a high negative voltage potential with respect to the nozzle body which is grounded. The workpiece on which cutting is to be performed is also connected to the ground of the power source (a higher potential than the negatively charged electrode). The nozzle assembly also comprises a series of fluid outlets which couple between, a gas source and the inner chamber of the nozzle where the electrode is disposed. The outlets surround the electrode so that when gas delivery is initiated, an annular flow of gas is delivered around the electrode in the nozzle assembly. The electromagnetic field surrounding the electrode ionizes the gas thereby forming a plasma arc. Typically, when plasma operation begins the nozzle is coupled to the ground of the power source such that a plasma arc is formed between the electrode and the nozzle. The plasma can be transferred to arc between the electrode and the workpiece rather than the electrode and the nozzle by bringing the nozzle assembly close to the workpiece and disconnecting the nozzle from ground.
In the prior art, a resistor coupled in series with an electro-mechanical relay between the nozzle and the power supply ground was used to disconnect the nozzle from ground when the torch was brought close enough to the workpiece to begin cutting operations. Such a configuration is shown in FIG. 1. In FIG. 1, the power supply is shown at 12, the electrode at 14 and the nozzle at 16. The nozzle 16 is coupled to the ground of the power through resistor 18 and relay K1. The workpiece 20 is also shown coupled to the ground of the power supply 12. When plasma operation is first initiated, relay K1 is closed providing a path for the current in the plasma arc from the negative side of the power supply 12 to the electrode 14, through the plasma arc to nozzle 16, and through resistor 18 and relay K1 back to the ground of the power supply. As the output of power supply 12 is increased, the voltage across resistor 18 increases. When the voltage drop across the resistor 18 reaches a threshold level, it causes the arc to transfer to the workpiece at which time relay K1 can be opened such that the only path to ground is through the workpiece 20.
When the plasma arc is coupled between the electrode 14 and the nozzle 16, it is called pilot arc operation. When the plasma arc is coupled between the electrode 14 and the workpiece 20, it is called cutting arc or torch operation. To return from cutting operation back to pilot arc operation, the current output by the power 12 is reduced to the point where the voltage drop across resistor 18 causes relay K1 to close again providing a path for the current to ground through the nozzle 16. When the voltage drop across resistor 18 is small enough, the plasma arc again attaches to the nozzle.
In this prior art method of transferring the plasma arc between the nozzle and the workpiece, the relay K1 causes the operation to switch abruptly such that the entire power flow is transferred from the workpiece to the nozzle or vice versa instantaneously. A smooth and/or linear transfer of power is preferable for safety reasons as well as performance reasons. Since the plasma beam in a plasma torch can reach temperatures exceeding 20,000.degree. Kelvin, an abrupt transfer of the plasma arc from the workpiece to the nozzle while the torch is still at full power, could seriously damage or even melt the nozzle.
Therefore, it is an object of the present invention to provide a plasma arc transfer control circuit for a plasma arc torch in which the transfer from pilot arc operation to cutting operation and vice versa is accomplished in a smooth and linear fashion.
It is a further object of the present invention to provide a plasma arc transfer control circuit utilizing feedback of the voltage differential between the workpiece and the nozzle to control plasma arc transfer.
It is yet another object of the present invention to provide a plasma arc transfer control circuit which automatically transfers the plasma arc from cutting operation to pilot arc operation a the workpiece is removed from the vicinity of the torch.
It is one more object of the present invention to provide a plasma torch in which power delivery to the electrode is automatically reduced when the plasma arc is transferred from cutting operation to pilot arc operation.
It is a further object of the present invention to provide an improved power transfer circuit.